The South Downs National Park Authority (SDNPA), which covers 627 square-miles across Sussex and Hampshire, has started its operational activities.
Legal powers have been taken up by the authority, which will carry out duties regarding planning within the park
The park was given approval in 2009 and was officially designated a year ago. It took the Government a decade to designate the area as a National Park.
The area stretches more than 100 miles from Winchester in the west to Eastbourne in the east, and includes rolling chalk uplands, river valleys and wooded greens.
The SDNPA expects to deal with 4,000 planning applications a year which will make it one of the largest planning authorities in the country.
Although there will be 15 local authorities to carry out the day-to-day planning within the park's boundaries the overall decisions will be taken by SDNPA.
SDNPA chairwoman Margaret Paren said: "This is a long-awaited and historic day for everyone who loves the South Downs.
"Given the unique character of the National Park, over the past year we have been developing our approach for the future in close consultation with local communities, environmental groups and landowners.
"As the new Authority, we have statutory purposes and socio-economic duties which we will be working hard to fulfil.
"By working together, we can all protect and enhance the National Park for residents and visitors today and future generations to come."
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